Mass balance of nitrogen in constructed mangrove wetlands receiving ammonium-rich wastewater: Effects of tidal regime and carbon supply

A series of computer-controlled mangrove tide-tanks planted with Kandelia candel was constructed to investigate the removal and transformation of ammonium–nitrogen under two tidal regimes: (i) 12-h wet/12-h dry (long tidal regime) and (ii) 6-h wet/6-h dry/6-h wet/6-h dry daily (short tidal regime). All tanks were irrigated with NH₄Cl solution for nine water cycles (each cycle lasted for 5 weeks) at an amount of around 2.1 g NH₄Cl (equivalent to 0.52 g N) per tank per cycle. During the experiment, total Kjeldahl nitrogen (TKN), inorganic nitrogen (N) (NH₄⁺–, NO₂^?–, and NO₃^?–N) and carbon were completely removed by the mangrove system. The added NH₄⁺–N was not detected in tidal water or accumulated in sediment. The mass balance of nitrogen showed that the discharge of ammonium-rich wastewater to mangrove wetlands enhanced microbial nitrogen transformation, particularly nitrification and denitrification processes, with 15–30% of the total nitrogen inputs returned to atmosphere as N₂ gas. Growth of K. candel and macroalgae was stimulated by ammonium addition, and up to 3 and 7% of total N inputs were assimilated in plant and algal tissues, respectively. Constructed mangrove wetlands with short tidal regime had higher numbers of nitrifiers and significantly lower content of ammonium that those with long tidal regime. On the other hand, higher populations of denitrifiers and lower nitrate were found in mangroves with long tidal regime and with glucose addition.     

Investigation on the Assimilation of Nitrogen by Maize Roots and the Transport of Some Major Nitrogen Compounds by Xylem Sap

The uptake and assimilation of nitrate and ammonia have been studied in Zea mays. Nitrogen-starved maize roots are capable of accumulating a potential capacity for nitrogen uptake and assimilation. Reestablishment of nitrogen supply leads to intense uptake, reaching 154 % of the reference variant level after 24 hours when nitrate is supplied, and 121 % when ammonia is supplied. After 24 hours the insoluble nitrogen fraction accounts for 80, 54 and 55 % of the total taken up in the PK + NO₃^-, PK + NH₄⁺ and NPK variants respectively.     

The effect of some ammonium salts on nitrate reductase level,onin vivo nitrate reduction and on nitrate content in excisedpisum sativum roots

The effect of some ammonium salts on nitrate reductase (NR) level, onin vivo nitrate reduction and on nitrate content was followed in the presence of nitrate in the medium, under changing experimental conditions, in excisedPisum sativum roots, and their effect was compared with that of KNO₃, Ca(NO₃)₂ and NaNO₃ at 15 mM NO₃ ^- concentration, i.e. at a concentration which considerably exceeded the level of saturation with nitrate with respect to nitrate reductase. The effect of ammonium salts on NR level is indirect and changes from a positive one to a strongly negative one which is dependent on the time of action of the salt, on the presence of other cations, on pH of the solution of the ammonium salt and on the nature of the anion of the ammonium salt. A positive effect on the enzyme level can be observed in the presence of other cations than NH₄ ⁺ at suitable concentrations of those ammonium salts, the solutions of which have their pH values in the acid region (i.e. NH₄H₂PO₄, (NH₄)₂SO₄ and NH₄NO₃). However their positive effect is independent of the presence of NH₄ ⁺ ions, and it is obviously the result of an increased concentration of H⁺ ions. A clear-cut negative effect on NR level can be observed after 24 h in one-salt NH₄NO₃ solution where NH₄ ⁺ is not balanced with other cations and thus certainly can adversely influence many metabolic processes, and in the solutions containing neutral (pH 6.2) and dibasic ammonium phosphates in which dissolved undissociated ammonia [(NH₃). (H₂O) which can also affect many metabolic processes incl. proteosynthesis] probably has a toxic influence. Thein vivo nitrate reduction is always depressed in excised pea roots in the presence of ammonium salts in the medium, regardless of the level of nitrate reductase. Under the described conditions, no relationship could be established between the enzyme level and the so-called metabolic NO₃ ^- pool (i.e. NO₂ ^- production under anaerobic conditions), nor between NR level and the total nitrate content in the roots. One-salt solutions of NaNO₃, Ca(NO₃)₂ and KNO₃ exert different effects on the level of nitrate reductase and on the content of NO₃ ^- in the roots, but the in vivo NO₃ ^- reduction shows the same trend as NR level in the roots influenced by these salts. Cl^- ions, supplied in NH₄C1, depress both NR level and NO₃ ^- content in the roots at higher concentrations, but they do not significantly affect the in vivo nitrate reduction in comparison with other ammonium salts. These results indicate that NR level,in vivo nitrate reduction, and nitrate uptake can be regulated in pea roots independently of each other.     

Seasonal nutrient fluxes variability of northern salt marshes: examples from the lower St. Lawrence Estuary

This study presents the tidal exchange of ammonium, nitrite + nitrate, phosphate and silicate between two salt marshes and adjacent estuarine waters. Marsh nutrient fluxes were evaluated for Pointe-au-Père and Pointe-aux-épinettes salt marshes, both located along the south shore of the lower St. Lawrence Estuary in Rimouski area (QC, Canada). Using nutrients field data, high precision bathymetric records and a hydrodynamic numerical model (MIKE21-NHD) forced with predicted tides, nutrients fluxes were estimated through salt marsh outlet cross-sections at four different periods of the year 2004 (March, May, July and November). Calculated marsh nutrient fluxes are discussed in relation with stream inputs, biotic and abiotic marsh processes and the incidence of sea ice cover. In both marshes, the results show the occurrence of year-round and seaward NH₄ ⁺ fluxes and landward NO₂ ⁻ + NO₃ ⁻ fluxes (ranging from 9.06 to 30.48 mg N day⁻¹ m⁻² and from −32.07 to −9.59 mg N day⁻¹ m⁻², respectively) as well as variable PO₄ ³⁻ and Si(OH)₄ fluxes (ranging from −3.73 to 6.34 mg P day⁻¹ m⁻² and from −29.19 to 21.91 mg Si day⁻¹ m⁻², respectively). These results suggest that NO₂ ⁻ + NO₃ ⁻ input to marshes can be a significant source of NH₄ ⁺ through dissimilatory nitrate reduction to ammonium (DNRA). This NH₄ ⁺, accumulating in marsh sediment rather than being removed through coupled nitrification–denitrification or biological assimilation, is exported toward estuarine waters. From average P and Si tidal fluxes analysis, both salt marshes act as a sink during high productivity period (May and July) and as a source, supplying estuarine water during low productivity period (November and March).     

In situ studies of algal biomass in relation to physicochemical characteristics of the Salt Plains National Wildlife Refuge, Oklahoma, USA

This is the first in a series of experiments designed to characterize the Salt Plains National Wildlife Refuge (SPNWR) ecosystem in northwestern Oklahoma and to catalogue its microbial inhabitants. The SPNWR is the remnant of an ancient ocean, encompassing ~65 km² of variably hypersaline flat land, fed by tributaries of the Arkansas River. Relative algal biomass (i.e., chlorophyll concentrations attributed to Chlorophyll-a-containing oxygenic phototrophs) and physical and chemical parameters were monitored at three permanent stations for a one-year period (July 2000 to July 2001) using a nested block design. Salient features of the flats include annual air temperatures that ranged from -10 to 40°C, and similar to other arid/semi-arid environments, 15–20-degree daily swings were common. Shade is absent from the flats system; intense irradiance and high temperatures (air and sediment surface) resulted in low water availability across the SPNWR, with levels of only ca. 15 % at the sediment surface. Moreover, moderate daily winds were constant (ca. 8–12 km h^-1), sometimes achieving maximum speeds of up to 137 km h^-1. Typical of freshwater systems, orthophosphate (PO₄ ^3-) concentrations were low, ranging from 0.04 to <1 μM; dissolved inorganic nitrogen levels were high, but spatially variable, ranging from ca. 250–600 μM (NO₃ ^- + NO₂ ^-) and 4–166 μM (NH₄ ⁺). Phototroph abundance was likely tied to nutrient availability, with high-nutrient sites exhibiting high Chl-a levels (ca. 1.46 mg m^-2). Despite these harsh conditions, the phototrophic microbial community was unexpectedly diverse. Preliminary attempts to isolate and identify oxygenic phototrophs from SPNWR water and soil samples yielded 47 species from 20 taxa and 3 divisions. Our data indicate that highly variable, extreme environments might support phototrophic microbial communities characterized by higher species diversity than previously assumed.     

九龙江河口区养虾塘沉积物-水界面营养盐交换通量特征

通过对九龙江河口区陆基养虾塘水样和沉积物样品采集分析及结合室内模拟实验,探讨了虾塘在不同养殖阶段沉积物-水界面营养盐通量时间变化特征及其主要影响因素。虾塘沉积物向上覆水体释放NO_x~--N(NO_2~--N和NO_3~--N)、NH_4~+-N和PO_4~(3-)-P能力均呈现随养殖时间推移而降低的特征。沉积物在养殖中期和后期分别呈现对上覆水体NO_x~--N和PO_4~(3-)-P的吸收现象,但总体表现为释放(平均通量分别为(1.87±1.15)、(1.58±0.52)mg m~(-2)h~(-1)和(1.22±0.62)mg m~(-2)h~(-1))。沉积物-水界面溶解无机氮交换以NH_4~+-N为主(沉积物平均释放通量为(46.18±13.82)mg m~(-2)h~(-1))。沉积物间隙水与上覆水间的营养盐浓度差(梯度)及温度对上述交换通量的时间动态特征具有重要调控作用。研究结果表明养殖初期或中期沉积物较高的无机氮(尤其是NO_2~--N和NH_4~+-N)释放是养殖塘水质恶化的一个极具潜力的污染内源,可能会对虾的健康生长产生负面效应,控制沉积物无机氮释放是养虾塘养殖初期和中期重要的日常管理活动之一。     

Removal of nitrogen by heterotrophic nitrification–aerobic denitrification of a novel halotolerant bacterium Pseudomonas mendocina TJPU04

Excess inorganic nitrogen in water poses a severe threat to enviroment. Removal of inorganic nitrogen by heterotrophic nitrifying–aerobic denitrifying microorganism is supposed to be a promising and applicable technology only if the removal rate can be maintained sufficiently high in real wastewater under various conditions, such as high concentration of salt and wide range of different nitrogen concentrations. Here, a new heterotrophic nitrifying–aerobic denitrifying bacterium was isolated and named as Pseudomonas mendocina TJPU04, which removes NH₄⁺-N, NO₃⁻-N and NO₂⁻-N with average rate of 4.69, 5.60, 4.99 mg/L/h, respectively. It also maintains high nitrogen removal efficiency over a wide range of nitrogen concentrations. When concentration of NH₄⁺-N, NO₃⁻-N and NO₂⁻-N was up to 150, 150 and 50 mg/L, 98%, 93%, and 100% removal efficiency could be obtained, respectively, after 30-h incubation under sterile condition. When it was applied under non-sterile condition, the ammonia removal efficiency was slightly lower than that under sterile condition. However, the nitrate and nitrite removal efficiencies under non-sterile condition were significantly higher than those under sterile condition. Strain TJPU04 also showed efficient nitrogen removal performance in the presence of high concentration of salt and nitrogen. In addition, the removal efficiencies of NH₄⁺-N, NO₃⁻-N and TN in real wastewater were 91%, 52%, and 75%, respectively. These results suggest that strain TJPU04 is a promising candidate for efficient removal of inorganic nitrogen in wastewater treatment.

     

Interactive effect of salt (NaCl) and nitrogen form on growth, water relations and photosynthetic capacity of sunflower (Helianthus annum L.)

The effects of the ammonium (NH₄⁺) and nitrate (NO₃^-) forms of nitrogen and NaCl on the growth, water relations and photosynthesis performance of sunflower (Helianthus annuus L.) were examined under glasshouse conditions. Eight-day-old plants of cv. Hisun 33 were subjected for 21 days to Hoagland's nutrient solution containing 8 mol m^-3N as NH₄⁺or NO₃^-, and salinised with 0, 60, or 120 mol m^-3NaCl. Fresh weights of shoots and roots, and leaf area of NO₃^-supplied non-salinised plants were significantly greater than those of NH₄⁺-supplied non-salinised plants. But addition of NaCl to the rooting medium of these plants had more inhibitory effect on the growth of NO₃^--supplied plants than on NH₄⁺-supplied plants. Both leaf water and osmotic potentials of plants grown with NH₄⁺were lower than those of plants given NO₃^-under both non-saline and saline conditions. Chlorophylls a and b concentrations were higher in plants grown with NH₄⁺than N0₃^--supplied plants at the lower two levels of salinisation. The rate of photosynthesis in plants was considerably higher in non-salinised plants grown with NO₃^-than with NH₄⁺, but with increase in salinisation the photosynthesis rate decreased in NO₃^--supplied plants, but not in those given NH₄⁺. The rate of transpiration was increased significantly by salinisation in NO₃^--supplied plants, but not consistently so in NH₄⁺-supplied plants. The stomatal conductances were much higher in plants given NO₃^-than with NH₄⁺when grown under non-saline conditions, but not when salinised. As a consequence, water-use efficiency in NO₃^--supplied control plants was better than in NH₄⁺-supplied under non-saline conditions, but worse under saline conditions. The different forms of nitrogen and the addition of NaCl to the growing medium did not affect the relative intercellular concentrations of CO₂ (C_i/C_a). Overall, the NH₄⁺form of nitrogen inhibited the growth of sunflowers under non-saline conditions, but NO₃^-and NaCl interacted to inhibit growth more than did NH₄⁺under saline conditions.     

The influence of ammonia on the growth and photosynthesis of Ruppia drepanensis Tineo from Doñana National Park (SW Spain)

In a laboratory experiment, Ruppia drepanensis Tineo seedlings from a brackish marsh in Southern Spain were grown at 20 and 30 °C, at three different nitrogen levels. These levels were obtained by the addition of a slow release fertilizer (23% NH₄NO₃ by weight) to a sediment mixture of sand and clay (3:1). Several morphometric parameters were recorded during the first five weeks of the experiment, and photosynthesis and respiration were measured after 7 weeks of growth. Results showed a significant reduction of growth and development with increasing nitrogen and temperature levels. Dark respiration increased strongly at high nitrogen levels. At the same time, net photosynthesis at 250 and 500 µE m^-2 s^-1, Pm, Km and LCP were not affected by either factor. We attribute these phenomena to ammonia toxicity, since relatively high total ammonia (NH₃ + NH_f4 ^p+) levels were found in the interstitial water.     

Nitrification and Denitrification in Lake and Estuarine Sediments Measured by the 15N Dilution Technique and Isotope Pairing

The transformation of nitrogen compounds in lake and estuarine sediments incubated in the dark was analyzed in a continuous-flowthrough system. The inflowing water contained ¹⁵NO₃^-, and by determination of the isotopic composition of the N₂, NO₃^-, and NH₄⁺ pools in the outflowing water, it was possible to quantify the following reactions: total NO₃^- uptake, denitrification based on NO₃^- from the overlying water, nitrification, coupled nitrification-denitrification, and N mineralization. In sediment cores from both lake and estuarine environments, benthic microphytes assimilated NO₃^- and NH₄⁺ for a period of 25 to 60 h after darkening. Under steady-state conditions in the dark, denitrification of NO₃^- originating from the overlying water accounted for 91 to 171 μmol m^-2 h^-1 in the lake sediments and for 131 to 182 μmol m^-2 h^-1 in the estuarine sediments, corresponding to approximately 100% of the total NO₃^- uptake for both sediments. It seems that high NO₃^- uptake by benthic microphytes in the initial dark period may have been misinterpreted in earlier investigations as dissimilatory reduction to ammonium. The rates of coupled nitrification-denitrification within the sediments contributed to 10% of the total denitrification at steady state in the dark, and total nitrification was only twice as high as the coupled process.     

硝态氮异化还原机制及其主导因素研究进展

硝态氮(NO_3~-)异化还原过程通常包含反硝化和异化还原为铵(DNRA)两个方面,是土壤氮素转化的重要途径,其强度大小直接影响着硝态氮的利用和环境效应(如淋溶和氮氧化物气体排放)。反硝化和DNRA过程在反应条件、产物和影响因素等方面常会呈现出协同与竞争的交互作用机制。综述了反硝化和DNRA过程的研究进展及其二者协同竞争的作用机理,并阐述了在NO_3~-、pH、有效C、氧化还原电位(Eh)等环境条件和土壤微生物对其发生强度和产物的影响,提出了今后应在产生机理、土壤环境因素、微生物学过程以及与其他氮素转化过程耦联作用等方面亟需深入研究,以期增进对氮素循环过程的认识以及为加强氮素管理利用提供依据。     

鄂尔多斯台地盐沼滩涂湿地土壤细菌群落结构及特征

依据植被分类法将鄂尔多斯高原盐沼滩涂湿地划分为肉质耐盐草甸(B)、苔草草甸(C)、禾草草甸(D)和杂类草草甸(E)等4个植被亚型,并以盐沼裸地(A)为对照样地,共计5种盐沼滩涂湿地景观类型。运用高通量测序技术分别研究其土壤细菌群落结构特征、分布情况,以及土壤盐分与土壤细菌之间的关系。结果表明:(1) 5种盐沼滩涂湿地的土壤细菌样品共12213条OTUs,属于45个门,122个纲,365个目,663个科,1375个属,2882个种。(2)变形菌门(26.19%)是盐沼滩涂湿地平均相对丰度最高的门,其次为放线菌门(17.15%),绿弯菌门(12.62%),芽单胞菌门(Gemmatimonadetes,11.23%),拟杆菌门(Bacteroidetes,9.38%),酸杆菌门(Acidobacteria,8.83%)厚壁菌门(Firmicutes,2.96%);芽单胞菌纲中的未定细菌(norank＿c＿Gemmatimonadetes)是丰度最高的属,平均丰度为5.75%。(3)鄂尔多斯盐沼滩涂湿地自西南向东北,空间位置相近的土壤细菌群落结构具有更大的相似性,随着土壤盐分变化,变形菌门相对...     

Effect of nitrogen form and phosphorus source on the growth,nutrient uptake and rhizosphere soil property of Camellia sinensis L.

The effects of nitrogen form and phosphorus source on the growth, nutrient uptake and rhizosphere soil property of tea (Camellia sinensis L.) were investigated in a pot experiment. The experiment was performed with a compartmental cropping device, which enables the collection of rhizosphere soil at defined distances from the root of tea plant. Nitrogen was supplied as nitrate or ammonium in combination with soluble phosphorus as Ca(H₂PO₄)₂ or insoluble P as rock phosphate. The leaf dry matter production of tea was significantly greater in the treatments with NH₄ ⁺ than NO₃ ^-, whereas dry matter production of root and stem was not significantly affected. Addition of phosphorus as either source did not influence the dry matter production. The concentrations of K in root, Mg and Ca in both the shoot and root supplied with NO₃ ^- were significantly higher than in NH₄ ⁺ and influence of P sources was minor. On the contrary, Al and Mn concentrations were significantly larger in NH₄ ^--fed plants which could be attributed to remarkably increased availability of Al and Mn caused by acidification of the rhizosphere soil (the first 1-mm soil section from the root surface) with NH₄–N nutrition. The concentration of N in shoot was also significantly higher in NH₄- than in NO₃-fed plants, indicating higher use efficiency of NH₄–N. Whatever the phosphate source, rhizosphere pH declined in ammonium compared to in nitrate treatment. The pH decrease was much larger when no P or soluble P were applied and reached 0.85–1.30 units which extended to 3–5 mm away from the root surface. Exchangeable acidity, content of exchangeable Al and Mn were also considerably higher in the rhizosphere soils of NH₄ ⁺ fed tea plants. Significant amounts of P dissolved from rock phosphate accumulated in rhizosphere of NH₄ ⁺, not NO₃ ^-, suggesting that the dissolution of rock phosphate was induced by the proton excreted by tea root fed with ammonium. With soluble P addition, shoot and root P concentrations were greater in NH₄ ⁺ than in NO₃ ^- treatment and it appeared that this difference could not be sufficiently explained by the available P content in soil which was only slightly higher in NH₄ ⁺ treatment. With rock phosphate addition, the shoot and root P concentrations were hardly affected by nitrogen form, although the available P content was much higher and accumulated in the rhizosphere soil supplied with ammonium. The reason for this was discussed with regard to the inter-relationship of Al with P uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同氮素水平对油麦菜栽培效果的综合评价分析

为探讨不同氮素水平的硝态氮与铵态氮对油麦菜(Lactuca sativa L.)生长及品质的影响,自主设计并制作了适合蔬菜水培的立体管道栽培系统,配制了NO3-：NH4+=10：0,NO3-：NH4+=7：1,NO3-：NH4+=7：3的营养液,通过模糊数学原理,采用层次分析法(AHP)确定品质的权重值,综合评价了不同氮素水平对油麦菜的栽培效果。结果表明,以NO3-：NH4+=7：3营养液的栽培效果最好,其次是NO3-：NH4+=7：1。当营养液的NO3-：NH4+=7：3时,油麦菜的产量最高,叶片叶绿素、可溶性糖、可溶性蛋白质含量最高,硝酸盐含量最少,其综合品质最好。层次分析法对油麦菜不同氮素水平营养液的栽培效果的综合评价有较好的适用性,有助于获得南方设施管道栽培油麦菜最佳的营养液配方。     

Contrasting nitrogen uptake by diatom and Phaeocystis-dominated phytoplankton assemblages in the North Sea

This paper documents ambient concentrations of nutrients in the Belgian coastal waters of the North Sea during the spring of 1996 and 1997. The paper elaborates the differences of uptake rates of oxidised nitrogen (NO₃⁻) and reduced nitrogen (NH₄ and urea) by Phaeocystis and diatoms. The nitrogen concentrations were dominated by NO₃⁻ with a maximum concentration of 30 μM (January 1997) and 40 μM (March 1996). In 1996, Phaeocystis dominated the spring biomass with a maximum of 521 μg C l⁻¹, while maximum diatom biomass was 174 μg C l⁻¹. In 1997, the maximum Phaeocystis spring biomass was 1600 μg C l⁻¹ and diatom maximum biomass was below 100 μg C l⁻¹. A maximum bacteria biomass of about 55 μg C l⁻¹ was observed in mid-May 1996. The maximum nitrogen uptake rates were recorded during spring and were dominated by NO₃⁻ (0.005 h⁻¹ in 1996 and 0.032 h⁻¹ in 1997). Maximum specific NH₄ uptake rates were between 0.005 h⁻¹ in May 1996 and 0.006 h⁻¹ in April 1997. The NO₃⁻ uptake rates displayed exponential decrease versus increasing ambient reduced nitrogen concentrations (ammonium and urea), whereas the reduced nitrogen uptake increased but never compensated the decreased nitrate uptake. The NH₄ uptake kinetics of diatoms displayed lower v_max compared to Phaeocystis. Consequently, Phaeocystis showed ability to increase their NH₄ uptake capacity when more NH₄ became available while diatoms failed to do so, after ammonium had exceeded their saturation concentration (>1 μM). Although reduced nitrogen has a negative effect on the uptake of NO₃⁻, Phaeocystis have more advantage than diatoms on the uptake of ammonium. This might be contributing to the biomass domination shown by Phaeocystis over extended periods in spring.     

SHORT TERM UPTAKE OF NUTRIENTS BY ENTEROMORPHA PROLIFERA (CHLOROPHYCEAE)1

Rates of NH₄⁺ and NO₃^? uptake were determined by accumulation of ¹⁵N in plant tissue and by disappearance of nutrient from the medium. Agreement between rates calculated by the two methods was good, averaging 82.7% (SD = 15.8%) and 91.2% (SD = 13.7%) for NH₄⁺ and NO₃^? uptake, respectively. An average of 93.4 and 96.0% of added ¹⁵NH₄⁺ and ¹⁵NO₃^? was recovered from the medium and /or plant tissue at the end of the incubations. Both bacterial uptake and regeneration of NH₄⁺ may contribute to discrepancies between NH₄⁺ uptake rates calculated by ¹⁵N accumulation and disappearance of NH₄⁺ from the medium. The influence of tissue composition on uptake of NH₄⁺, NO₃^? and PO₄^3- by Enteromorpha prolifera (Müller) J. Agardh was examined. For NH₄⁺ uptake, V_max was 188 μmol NH₄^+. g dry wt^?1. h^?1 and K_s ranged from 9.3 to 13.4 μM, but there was no correlation between kinetic parameters and tissue nitrogen content. For NO₃^?, both kinetic parameters were higher for plants with low tissue nitrogen than for plants with high tissue nitrogen. Maximum rates were 169 and 75.4 μmol NO₃^?. g dry wt^?1. h^?1, and K_s was 13.3 and 2.31 μM for low and high tissue nitrogen plants, respectively. Estimates of uptake in the field suggested that NH₄⁺ accounted for 65% and NO₃^? for up to 35% of total nitrogen uptake during the summer. Nutrient uptake rates of field-collected plants also indicated that E. prolifera in Yaquina Bay, Oregon was not likely to have been nitrogen-limited, but may have been phosphorus-limited.     

The influence of NO3 - and NH4 + nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO₃ ^- or NH₄ ⁺ nutrition were investigated using either ¹⁴C or ¹⁵N techniques. Greater allocation of ¹⁴C to amino-N fractions occurred at the expense of allocation of ¹⁴C to carbohydrate fractions in NH₄ ⁺-compared to NO₃ ^--fed plants. The [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO₃ ^--compared to 12 mM NH₄ ⁺-fed wheat. In both 4 mM and 12 mM N-fed maize the [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO₃ ^--compared to NH₄ ⁺-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO₃ ^--and the other in NH₄ ⁺-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of ¹⁴N and ¹⁵N within separate sets of plants confirmed that NH₄ ^--fed plants accumulated more amino-N compounds than NO₃ ^--fed plants. Wheat roots supplied with ¹⁵NH₄ ⁺ for 8 h were found to accumulate ¹⁵NH₄ ⁺ (8.5 g ¹⁵N g^-1 h^-1) whereas in maize roots very little ¹⁵NH₄ ⁺ accumulated (1.5 g ¹⁵N g^-1 h^-1)It is proposed that the observed accumulation of ¹⁵NH₄ ⁺ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH₄ ⁺, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C₄ maize plants ensure greater carbon availability to the root than in the C₃ wheat plants. These differences in carbon and nitrogen partitioning between NO₃ ^--and NH₄ ⁺-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO₃ ^- and NH₄ ⁺ nutrition.     

Studies on nitrate reductase activity in Laminariadigitata (Huds.) Lamour. I. Longitudinal and transverse profiles of nitrate reductase activity within the thallus

The distribution of the enzyme nitrate reductase (NR) within the thallus of the brown alga Laminaria digitata (Huds.) Lamour is described for plants sampled from the east coast of Scotland in May and June when growth rates are at a maximum. Highest NR activities (≈ 0.2 μmol NO₃^? reduced·g^?1 wet wt·h^?1) occurred in the mature blade. NR activities declined towards the basal meristematic region of the blade. Activities in the stipe and holdfast were also low, being between 0.05 and 0.07 μmol NO₃^? reduced·g^?1 wet wt·h^?1. The activities of the enzyme glutamine synthetase (GS), which is important in the assimilation of NH₄⁺, showed a similar distribution within the blade to those of NR.The transverse profile of NR activity in the stipe exhibited a decline from the outer to the inner tissues. Maximum activities (0.13 μmol NO₃^? reduced·g^?1 wet wt·h^?1) occurred in the meristoderm, while those of the cortex and medulla were 0.04 and 0.01 μmol NO₃^? reduced·g^?1 wet wt·h^?1 respectively.These data indicate that most of the NO₃^? assimilation occurs in the mature blade rather than in the meristematic tissue where there is a high nitrogen demand for growth. The data are consistent with the maintenance of meristematic growth by the internal transport of organic nitrogen from the mature blade.     

UPTAKE OF INORGANIC NITROGEN BY CODIUM FRAGILE SUBSP. TOMENTOSOIDES (CHLOROPHYTA)1

The uptake of nitrate, nitrite and ammonium by Codium fragile subsp. tomentosoides (van Goor) Silva was measured at different combinations of temperature (6–30 C) and irradiance (0–140 μEin.m-^2. s^-1). Uptake of all three forms of N was greater at 12–24 C than at 6 and 30 C. Although uptake was stimulated by light, saturation occurred at relatively low irradiance (7–28 μEin m^-2 s^-1, depending on the N source and temperature). The Michaelis-Menten uptake constants (V_max K)varied with temperature. V_max was greatest at intermediate temperatures and K was lowest at lower temperatures. The V_maxfor NH₄⁺ was higher and the K, for NH₄⁺was lower than those for NO₃^-_- and NO₂^-_-. Codium was capable of simultaneously taking up all three forms of inorganic N although the presence of NH₄⁺ reduced the uptake of both NO₃^-_- and NO₂^-_-. The results of this study indicate that part of the ecological success of Codium in a N-limited environment may be due to its N uptake capabilities.